1. Field of the Invention
This invention relates to an oxygen sensor deterioration-detecting device for an internal combustion engine having oxygen sensors provided in the exhaust system for detecting oxygen concentration in exhaust gases at respective locations upstream and downstream of an catalytic converter in the exhaust system.
2. Prior Art
In general, the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine is controlled in response to the concentration of oxygen in exhaust gases from the engine detected by an oxygen sensor (hereinafter referred to as the "O2 sensor") such that the air-fuel ratio of the mixture becomes equal to a desired value.
The O2 sensor is liable to change in characteristics (internal resistance, electromotive force, and response time), due to its deterioration caused by heat and the like. The use of an O2 sensor which is deteriorated in characteristics, adversely affects the accuracy of air-fuel ratio control.
To overcome this inconvenience, various proposals have been made, which include additionally providing an O2 sensor in the exhaust system at a location downstream of a catalytic converter in the exhaust system, in order to compensate for undesirable changes in characteristics of the air-fuel ratio feedback control performed in response to an output from the O2 sensor upstream of the catalytic converter, whereby the air-fuel ratio feedback control is carried out with high accuracy. According to this proposal, in controlling the air-fuel ratio of the mixture supplied to the engine to a desired value in a feedback manner responsive to the output from the upstream O2 sensor, a control amount used in the air-fuel ratio feedback control is increased or decreased for correction based on an output from the downstream O2 sensor, to thereby compensate for deviation of the actual air-fuel ratio control from a value to which the air-fuel ratio of the mixture is to be controlled, due to deterioration of the upstream O2 sensor. However, this proposed solution has a disadvantage that when the upstream O2 sensor has heavily deteriorated beyond the limit of the above-mentioned compensation, it will result in degraded exhaust emission characteristics of the engine.
To overcome this disadvantage, it has been proposed, e.g. by Japanese Provisional Patent Publication (Kokai) No. 4-72438, to detect deterioration of an upstream O2 sensor based on a correction value which corrects the above-mentioned control amount, and a repetition period of inversion of an output from the upstream O2 sensor, to thereby alert the driver for replacement of the upstream O2 sensor with a new one when it is detected to be deteriorated, so as to prevent the engine from being operated with degraded exhaust emission characteristics.
However, the above proposed method of detecting the deterioration of the O2 sensor has a disadvantage that when the engine is normally operating, the output from the upstream O2 sensor is inverted periodically, but when the engine suffers from a certain abnormality, the control of the air-fuel ratio of the mixture supplied to the engine cannot be accurately carried out, which adversely affects the repetition period of inversion of the output from the upstream O2 sensor, leading to a possible erroneous determination that the upstream O2 sensor is deteriorated, though it is actually normally functioning.